1. Field of Invention
This invention relates generally to the fabrication of semiconductor devices and more particularly to a method of forming metallization patterns in a dielectric layer of a substrate for a photovoltaic cell by laser scanning selected regions of the substrate in an atmosphere substantially free of chemical etching substances The dielectric layer may consist of a glass or a silicon nitride layer.
2. Prior Art
Known semiconductor devices, such as photovoltaic cells, frequently comprise a plurality of conductive metallic segments attached to the front surface of a selected substrate for conducting current between various regions of the device. In the fabrication of such semiconductor devices, a plurality of relatively involved processing steps are typically required to prepare the surface of the substrate for metallization formation of the front surface conductors. These processing steps are required because the front surface of the substrate of conventional semiconductor devices is typically coated with a layer of material to which conductive materials deposited by known methods, e.g., the immersion plating process disclosed in U.S. Pat. No. 4,321,283, will not readily adhere. This front surface layer is typically a dielectric layer, e.g., a glass or silicon nitride layer. The glass and silicon nitride layers provide hard protective coatings for the underlying silicon substrate.
Conventionally, metallization patterns are formed by selectively removing portions of the dielectric front surface layer using known photoresist and etching methods. Selected portions (depending on the process used) of the front surface of the substrate define the regions to which conductors will be attached Examples of known solar cell photoresist and etching methods are taught in U.S. Pat. Nos. 4,451,969, 4,557,037, and 4,612,698.
Prior known methods of forming metallization patterns are not entirely satisfactory because they involve a relatively large number of processing steps. For instance, in U.S. Pat. No. 4,451,969, relating to the production of solar cells, the following steps are required to prepare a photovoltaic cell silicon substrate for metal plating:
(a) forming a polysilazane coating on a first surface of the substrate having a junction in the substrate adjacent the first surface;
(b) covering the polysilazane coating with a photoresistant material;
(c) exposing the photoresist coating to radiant energy through a mask defining a two-dimensional pattern;
(d) chemically developing the photoresist so that selected portions of the photoresist are removed according to the two-dimensional pattern;
(e) etching away those portions of the polysilazane coating not covered by the photoresist; and
(f) heating the silicon substrate at a temperature and for a time sufficient to:
(1) effect removal of said photoresist material by pyrolysis, and PA1 (2) modify said polysilazane coating so that it has a relatively low etch rate.
Since the cost of semiconductor devices such as solar cells is dependent to a major extent upon the number of required fabrication steps, it is clearly desirable to be able to form metallization patterns using a process having fewer steps than the foregoing process.
Lasers have been used to remove or alter the microstructure of various layers on substrates of semiconductor devices, in part, as a method of reducing the number of steps involved in fabricating the device. For instance, lasers have been used in conjunction with etchants to remove selected portions of semiconductor materials, as taught, for instance, in U.S. Pat. Nos. 4,331,504, 4,401,367, 4,478,677, 4,490,210, and 4,490,211. Lasers have also been used to alter the microstructure of portions of semiconductor materials to facilitate subsequent removal of the laser-scanned portions by specific surface etchants, as disclosed in U.S. Pat. Nos. 4,335,295 and 4,415,383. It also is known to cut holes by laser beam in thin films such as silicon nitride, copper and other conductor and semiconductor materials, as shown, for example, by U.S. Pat. Nos. Re: 27,772, 4,044,222 and 4,081,653. Lasers have also been used to anneal predetermined areas of amorphous or polycrystalline semiconductor materials into single crystal areas, as disclosed by U.S. Pat. No. 4,388,145.
Prior known methods of removing or altering the microstructure of electrical conductor or semiconductor materials using lasers are not directed to the production of metallization patterns in dielectric layers on silicon solar cells. Additionally, some of the known laser processing methods require the use of one or more surface-reactive etching agents. The use of such agents adds to the cost of fabrication of semiconductor devices and often presents additional storage and waste disposal problems.
A method of removing selected portions of the silicon nitride layer of a semiconductor substrate so as to expose the underlying silicon substrate using a laser beam in an environment that is substantially free of chemical etching substances is disclosed in U.S. Patent Application Ser. No. 387,854, filed on July 31, 1989 in the name of Hanoka et al (Attorney's Docket No. MTA-73). Although functional solar cells were fabricated in accordance with the Hanoka et al. process, it has been determined that conductors attached to the exposed portions of the silicon substrate do not always adhere as securely and as permanently as might be desired. It appears that a small amount of silicon nitride tends to remain or is redeposited on the portions of the silicon substrate exposed by laser removal of the selected portions of the silicon nitride layer. This residual silicon nitride is believed to adversely affect the adhesion of the conductors to the substrate.